The invention relates to a method for measuring the mass flow of a particulate solid which is transported by means of a gas, a measuring region being defined in an electromagnetic field, and the electromagnetic radiation reflected by the solid(s) being evaluated. Moreover the invention relates to an arrangement for performing the method, which has a device for producing an electromagnetic field in which a measuring region is defined, and with an evaluation device, connected to a detector, for at least the electromagnetic radiation reflected from the solid(s).
A series of different methods and devices are known for the determination of the mass flow (also termed “throughput”) of a solid which preferably passed through a tube with a transport gas, for example, air.
“Mass flow” will always be understood hereinafter the transport of a weight unit of a material in a given time unit: for example, Kg/s or t/h.
The solid is preferably comminuted or milled, so that as a rule it is present as a powder or dust. It can also, however, naturally have a granular appearance, such as is the case of cereals. As the electromagnetic waves, microwaves, visible light, or infrared can be used.
In all the measurement methods known heretofore, which use electromagnetic waves for mass flow determination, on the one hand the concentration, and in addition the speed, of the solid are measured. For concentration measurements the damping of the amplitude of an electromagnetic wave is frequently determined; for speed measurements, the frequency shift due to the Doppler effect is frequently used. Both measurement results are then multiplied together to give the mass flow. These mass flow measuring devices thus consist in principle of two measuring devices.
Methods and devices which operate according to this principle are known from, for example, WO90/03668, Patent Abstracts of Japan, Vol. 8, No. 109 (P-275), 22 May 1984, JP-A-59 019814, and U.S. Pat. No. 4,580,441.
From U.S. Pat. No. 5,550,537, it is known to determine the concentration of the transported solid using the reflected energy or power. The flow speed measurement takes place by measurement of the frequency shift of the reflected radiation due to the Doppler effect.
Here also, two measurements and two mutually separated evaluations are necessary. A corresponding cost for the measuring and evaluation devices, and also a doubled possibility of error, are present.